HKK drought sensitivity results

Krishna Anujan

2025-01-10

Sensitivity of tree growth to precipitation in seasonally dry tropical forests using long-term dendrometer band measurements

Objective

Disentangling the effects of size, exposure and water availability on drought sensitivity of tree growth

Defining drought sensitivity

\(sensitivity_{tree} = \frac{drought.year.increment - mean.annual.increment_{tree}}{mean.annual.increment_{tree}}\)

Data

Variable Description
sensitivity Late wet/early dry dendrometer band measurements
exposure Crown Illumination index at previous dendroband census
size DBH at previous dendroband census
water Topographic wetness index

ENSO drought years were associated with growth declines across species…

…but we observed large variation in drought sensitivity within and among species

How well do species traits predict average sensitivity of species?

\(sensitivity_{tree}~ 1+ (1|Species)\)

Models

We ran 3 different models

  1. varying intercepts by species
    \(sensitivity_{tree} = \beta_{DBH}DBH_{0} + \beta_{TWI}TWI + \beta_{CII}CII_{0} + (1|Species)\)

  2. varying intercepts + predictors scaled within species
    \(sensitivity_{tree} = \beta_{DBH}DBH_sp_{0} + \beta_{TWI}TWI_sp + \beta_{CII}CII_sp_{0} + (1|Species)\)

  3. varying slopes + predictors scaled within species
    \(sensitivity_{tree} = \beta_{DBH}DBH_sp_{0} + \beta_{TWI}TWI_sp + \beta_{CII}CII_sp_{0} + (1+DBH+CII+TWI|Species)\)

Variation in sensitivity among individuals was explained more by crown exposure than size per se

Higher exposure was associated with more negative growth sensitivities across species

To corroborate these results we modelled growth occurrence as a binary response.

And found similar, stronger influence of exposure on growth occurrence

When modelled without exposure, the effect of size was more negative as exposure is strongly predicted by size

This implies mediation effects of size on exposure

Take-home messages

  1. In 2015, the stronger ENSO drought, deciduous and slow-growing species had stronger negative sensitivities.
  2. While two droughts decreased growth on most trees, more exposed trees were more negatively impacted than their less exposed counterparts
  3. While exposure effects were stronger than size effects on drought sensitivity, we see mediation effects.